Soldering of printed circuit boards and electronic parts to each other using a solder paste is usually carried out using a reflow furnace. A reflow furnace is constituted by a tunnel-shaped muffle having a preheating zone, a main heating zone, and a cooling zone inside it. A heating means is installed in the preheating zone and the main heating zone, and a cooling means is installed inside the cooling zone.
Heating means used in reflow furnaces include infrared heaters and hot air blowing heaters. With an infrared heater, infrared rays penetrate to the interior of crevices in a printed circuit board and electronic parts and melt the solder paste applied to portions to be soldered. However, infrared rays travel in a straight line, so an infrared heater has the problem that it cannot sufficiently heat portions to be soldered located in the shadows of electronic parts.
With a hot air blowing heater, hot air undergoes convection inside a muffle and penetrates to portions in the shadows of electronic parts and narrow crevices. Therefore, it has the advantage over an infrared heater that it can more uniformly heat the entirety of a printed circuit board than can an infrared heater, and it is much used today in reflow furnaces.
A hot air blowing heater installed in a conventional reflow furnace has hot air discharge ports with a large area along with hot air suction ports with a small area formed in the vicinity of the hot air discharge ports in the same plane. A large amount of hot air is blown out of the hot air discharge ports having a large area toward a printed circuit board such that a large area of the printed circuit board is heated at the same time.
It has been thought that a printed circuit board can be uniformly heated by blowing hot air in this manner from discharge ports having a large area. However, in recent experiments, it was found that a printed circuit board cannot be uniformly heated even if hot air is blown from discharge ports having a large area. Namely, after a printed circuit board is heated with hot air blown from hot air discharge ports having a large area, the printed circuit board advances and reaches the suction ports, where not only is the printed circuit board not struck by hot air, but suction of hot air into the suction ports causes the printed circuit board to cool. If the temperature profile in a reflow furnace having hot air discharge ports with a large area adjoining hot air suction ports having a small area in this manner is graphed, it is found that the temperature rises at the hot air discharge ports but decreases at the suction ports.
If the temperature in the preheating zone and the main heating zone fluctuates in this manner, a printed circuit board is not uniformly heated, and local overheating or underheating may take place to such an extent that electronic parts undergo thermal damage or solder paste is unmelted.
In light of such problems of reflow furnaces in which hot air is discharged from discharge ports having a large area, reflow furnaces have been proposed in which a large number of hot air discharge ports having a small area are provided along with a large number of hot air suction ports located in the vicinity of the hot air discharge ports (Patent Documents 1-7).
However, in the reflow furnaces of Patent Documents 1, 2, 4, and 5, for example, although the entirety of a printed circuit board can be uniformly heated because of discharge ports having a small area which are scattered, hot air which is blown from the large number of discharge ports is sucked in by suction ports in many locations since the suction ports are also scattered. As a result, the hot air which is discharged and the hot air which is sucked in strike against each other, and turbulence sometimes occurs in the muffle. The occurrence of turbulence in the muffle may cause heating irregularities, and in the case of an inert atmosphere using nitrogen gas, it may cause the oxygen concentration of the atmosphere to fluctuate due to infiltration of oxygen from outside the furnace.
The reflow furnace of Patent Document 3 has a series of discharge ports and a series of suction ports provided alongside the discharge ports, and therefore turbulence is not caused by hot air which is discharged from the discharge ports colliding with hot air which is sucked in by the suction ports. However, the reflow furnace of Patent Document 3 has a series of discharge ports arranged in the shape of an X or a Z or in a zigzag shape with respect to the direction of travel of a printed circuit board, and there are locations in which no discharge ports are present. Therefore, a printed circuit board is not adequately heated in locations where discharge ports are not present, and heating irregularities may end up developing.
Patent Documents 6 and 7 merely disclose hot air discharge nozzles which are provided so as to project from a suction plate having suction ports.
Patent Document 1: JP H02-303674 A1
Patent Document 2: JP 2001-144426 A1
Patent Document 3: JP 2001-144427 A1
Patent Document 4: JP 2001-326455 A1
Patent Document 5: JP 2002-198642 A1
Patent Document 6: JP 2002-331357 A1
Patent Document 7: JP 2003-332725 A1